A spider web is made of silk. Spiders have what is called spinneret glands. These glands produce the silk spiders use when they construct their webs and are located on the abdomen, usually on the bottom. The silk is very light as needed for the wind to carry it. Spiders can make sticky and nonsticky silk. The spider knows how to use both to build the most sturdy web possible. Spiders are careful when constructing webs to make it possible to stay on the parts that are not sticky to prevent themselves from becoming glued to their own web.
Spiders begin their webs by letting out a single thread that is carried by the wind. Using even the very slightest of breezes, the spider is patient as it waits for the thread to stick to a surface. This surface can be just about anything, such as walls, trees or windows. Once this is accomplished, the spider will crawl over it while adding additional layers of thread for the support of the rest of the web. The size of the web is determined by the size of the spider. The bigger the spider, the bigger the web. Different types of spiders design different types of webs, but generally the main thread is used as a support while the spider spins radials around it. Circles of thread in the centre finish the process giving it the familiar design of a spider web. The web is thoroughly tested by the spider for stability. If it proves to be unstable, the spider will usually build a new one, opting not to do any repairs.
Why Spiders Spin Webs
Not all spiders spin webs, but the main reason spiders spin webs is to catch their food. Insects easily get caught on the sticky threads of the web. The spider waits on the outside of the web for this to happen and once a movement is detected from the web, the spider will head in the direction of his next meal. Some spiders that do not spin webs in the normal manner will spin a web between their legs and carry it with them while they look for edible insects to pounce on. The unsuspecting prey is then wrapped in the web and bitten before becoming the spider's food for the day. This method of web construction requires a lot less energy from the spider.